The present invention relates to radio frequency amplifiers and particularly relates to a variable gain radio frequency amplifier stage.
Wireless communication systems enjoy significant popularity, finding widespread use in both developed and developing regions. Indeed, the very popularity of wireless communication systems spurs their development and advancement, driving system designers and service providers to devise ways of supporting more users within a finite radio frequency spectrum. Existing and pending wireless communications standards typically rely on frequency, code, or time-division multiplexing techniques that allow multiple portable communication device users to share the same frequencies within a given service area. Commonly, such access schemes benefit from each wireless device controlling their transmitted signal power to help minimize its interference with other active devices within a given wireless service area.
This power control approach presents portable device designers with significant challenges. For example, power-control techniques typically require transmit signal amplifier circuits that provide a range (often continuous) of transmit signal gain. This allows a controlling device (e.g., a cellular telephone) to transmit with a desired signal power based on adjusting the gain of one or more such transmit signal amplifier circuits. For example, as the portable device moves closer to a supporting base station, the wireless communications system may instruct, via control signaling, the portable device to reduce its transmit power. Essentially, in such wireless communications systems, active portable devices are controlled such that they transmit with the minimum necessary signal power at all times.
High signal fidelity requirements impose further challenges on designers of wireless communications transmitters. Many wireless communication standards impose strict adjacent channel power limitationsxe2x80x94a measure of unwanted signal power appearing in radio channels adjacent to the selected transmit channel. Digital modulation schemes, such as those used in GSM or PCS systems, typically require phase or frequency modulation in combination with amplitude modulation. The need for envelope modulation (amplitude) imposes a requirement for linear amplification of the transmit signal. As noted, this linear amplification function must usually support variable gain, so that the linear, radio frequency signal may be gain adjusted to comply with transmit signal output power control requirements.
Thus, modem wireless communication devices typically must meet the dual, sometimes contradictory requirements of providing flexible transmit signal output power control, while still maintaining good amplification linearity. A number of approaches exist for meeting these design challenges, and include transistor-based amplifier gain stages using differing topologies and differing gain control techniques. Some gain control arrangements adjust the bias signal applied to the transistor amplifiers, but this can have the disadvantage of changing the operating point of the transistors involved, and thus affecting linearity, particularly at the lowest levels of gain. At such low levels of gain, the transistor amplifiers may be at their lower limit of active mode operation, and can thus become significantly nonlinear.
Another approach to transistor amplifier gain control involves providing a transistor amplifier having a gain path and a shunt path, with the shunt path having no signal gain. In operation, a current steering mechanism splits current between the gain and shunt paths to provide a desired amount of signal gain. Because amplifier gain is not controlled through bias changes, this approach has the significant benefit of allowing operation of the transistor amplifier(s) at constant operating point, thereby aiding amplifier linearity. A xe2x80x9cGilbertxe2x80x9d cell represents such a configuration for a differential amplifier.
In a Gilbert cell, a differential transistor amplifier pair, each transistor amplifier sinks collector current through both a shunt path having no signal gain, and a gain path that provides signal gain. This is accomplished by placing a collector load (impedance) in the gain path but not in the shunt path. Therefore, the shunt path lacks any impedance across which an output voltage signal can be developed. Current steering circuits control the ratio of gain path and shunt path current to achieve a desired output signal gain.
A significant drawback of the Gilbert cell, and other amplifier topologies that employ current steering techniques to effect gain control is the inability to provide well-controlled minimum gain settings with such techniques. Theoretically, the minimum gain of such circuits is zero, with all of the transistor amplifier current flowing through the zero-gain shunt path. That is, at a given control voltage, the current steering mechanism blocks current from flowing through the gain path, diverting all amplifier current through the shunt path. However, this can result in unstable and widely varying amplifier characteristics as minimum gain is approached.
Accordingly, there remains a need for a radio frequency amplifier gain control technique, such that the gain-controlled amplifier exhibits well-controlled variable gain characteristics. Such characteristics include good amplifier linearity across the gain control range, low intermodulation distortion, and predictable, stable minimum gain characteristics.
The present invention provides both methods and apparatus for amplifier gain control based on a current steering arrangement that insures a well-defined minimum gain setting and good amplifier linearity over the gain control range. A transistor amplifier is configured with parallel output current paths, a gain path and a shunt path. The gain path includes an impedance element that develops an output signal voltage in response to the amplifier""s time-varying output current, while the shunt path is configured to be low-impedance. A current steering circuit determines how the amplifier output current splits between the gain and shunt paths, thereby controlling amplifier gain. At least one gain path primary transistor and shunt path primary transistor are disposed in the gain and shunt paths, respectively. Preferably, these transistors operate with inverse bias control, such that as one transistor turns on, the other turns off. A secondary transistor shares a common bias control with the shunt path primary transistor, but is disposed in the gain path with the gain path primary transistor. When the gain path primary transistor is biased fully off and the shunt path primary transistor is biased fully on at minimum amplifier gain, this secondary transistor is also biased on. This insures a well-controlled minimum amount of gain path current at the minimum amplifier gain setting.
The current steering arrangement may be adapted to both single-ended and differential amplifier topologies and is compatible with a wide range of specific transistor amplifier circuit implementations and bias arrangements. In preferred embodiments, the current steering arrangement is connected in cascade fashion in the collector or drain current path of the transistor amplifier or amplifiers. Gain control is accomplished through steering varying amounts of the amplifier output current through a gain path and a shunt path, rather than by varying amplifier operating voltage or amplifier bias. Thus, a variable gain amplifier operating in accordance with the present invention may employ a fixed bias at an optimum operating point, thereby exhibiting excellent linearity and low intermodulation distortion across the gain control range.
With excellent linearity and low intermodulation distortion characteristics, an amplifier in accordance with the present invention is particularly well suited for use in radio frequency communications apparatus. Such amplifiers may be implemented in a variety of process technologies, adapted for use at both intermediate and high frequencies. Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of exemplary embodiments of the invention in conjunction with the accompanying figures.